This invention relates to the art of decoding information signals received via an information transmission channel such as, for example, a radio, telephone or other communication circuit between distant points. More particularly the invention relates to a means for preventing the introduction of errors in decoding complementary trains of binary signals which are distorted by the transmission channel. The signals used in transmitting information via radio teleprinter channels are an example of complementary series of pulses. These signals are known as "mark" and "space" signals and are complementary because when the mark signal ends, the space signal begins and both the mark and space signals contain the same transmitted information. The radio transmitter emits power during both the mark and space signals but shifts the frequency of the transmitted signal slightly to generate mark and space radio signals at radio frequencies.
Mark and space signals transmitted via a radio communications channel are often distorted by multiple path conditions which are a phenomena of high frequency radio transmission. When multiple path conditions exist, a radio signal transmitted from point A to point B is received at point B via more than one path through the ionosphere. Since the longer paths have a longer transit time, at the point of reception a mark or space signal often appears to have been stretched in time. Also, the mark and space signals sometimes arrive at the reception point out of phase with one another which causes partial or complete phase cancellation of the signals.
A radio receiver converts mark and space signals at radio frequencies to mark and space signals at two audio frequencies having a predetermined frequency shift between the mark and the space audio signals. An apparatus, commonly called a converter, demodulator, or terminal unit, detects information from both the mark and space signals simultaneously. When phase cancellation occurs, no information or only partial information is detected by the converter. Both stretching of signals due to multiple path conditions and phase cancellation of signals causes an output device, such as a teleprinter which changes signals from a converter to the printed word, to make errors.
More particularly, converters of the prior art receive mark and space signals from a radio receiver in the form of audio tones at two different frequencies with the mark signals having one frequency such as 2125 Hz and the space signals having another frequency such as 2975 Hz. The mark and space tones are processed in separate channels which include a filter in each channel, with one filter designed to pass only tones at the frequency of the space signals and the other filter designed to pass only tones at the frequency of the mark signals. The tones passed by each filter are then coupled to a detector for demodulating, the tones thus resulting in a series of DC signals. The mark and space signals from the detectors are then summed. The combined signals may be coupled either directly to an output device such as a teleprinter. Conventionally a teleprinter includes an electromagnet whose armature movement controls the mechanical printing functions of the teleprinter. The armature responds to a mark pulse by moving in one direction and to a space pulse by moving in the opposite direction. In more sophisticated converters, the combined signal is filtered by a low pass filter which removes carrier frequencies and extraneous noise from the combined signal to produce DC pulses and, in some terminal units, the pulses from the filter are then coupled through a circuit for establishing a varying threshold level of incoming mark and space signals of varying signal strength, such as an automatic threshold corrector or assessor, and then coupled from this circuit to an output device such as a teleprinter.
Converters of the prior art which sum or combine information in the mark and space channels are not capable of providing error-free decoding of mark and space signals which are stretched by multiple path conditions or which are distorted by phase cancellation. When mark and space signals partially overlap one another, these converter units of the prior art detect information energy in both channels simultaneously. The overlapping information is cancelled in the process of summing or combining the mark and space signals. A teleprinter cannot make a correct mark or a space decision because the information needed to make a proper decision has been cancelled in the converter unit. Phase cancellation creates a gap in individual pulses causing one signal to appear as two signals. When operating on such distorted signals, an output device, such as a teleprinter, may print erroneous information or lose its synchronization with the transmitting device and, as a result of loss of synchronization, incorrectly print a number of characters even though only one character was distorted by multiple path propagation conditions.